The present invention relates to methods and apparatus for determining properties of given substances.
The invention relates to that type of method and apparatus which utilizes IR (infrared) spectroscopy by means of which it is possible to measure exceedingly small differential concentrations in various samples.
Along with the development of infrared analyzers, instruments based also upon molecular resonance absorption have already been utilized for many different purposes. Up to the present time the use of such instruments has in most cases required considerable experience inasmuch as conventional instruments of this type have components requiring numerous adjustments and zero settings. Such adjustments and settings are essential in order to achieve good results from instruments of the type referred to above.
It is common practice with such instruments to provide two measuring paths, the sample which is to be measured being situated along one of these paths while a reference substance is situated along the other of these paths, and the measurement is derived on the basis of the difference in absorption taking place along these two paths. Where radiation in the form of light, for example, is utilized, an unavoidable difficulty encountered with instruments of this type is in rendering both paths identical so that in all situations the cuvettes which receive the sample and reference substances will at the instant of measurement receive identical quantities of radiation. In this connection it is to be noted that in addition to providing identical quantities of radiation it is also essential to maintain the other operating parameters constant, and this identity of the operating conditions must be maintained under all operating variables and during aging of the instrument.
It has already been attempted to reduce the differences between the variables encountered along the two different measuring paths where the sample and reference are respectively situated, as by making these measuring paths as closely identical as possible, or mirror images of each other. However, with the continuous increase in the desire and requirement of sensitivity of the measurement, and insofar as the apparatus provides the possibility, the true problem is at the present time how to maintain the two paths mutually comparable with an extremely high degree of accuracy in spite of such factors as soiling, aging, and other sources.
In certain types of apparatus as referred to above, use is made of infrared spectroscopy with the intensity of the radiation which travels through the sample being compared with the intensity of the radiation travelling through the reference. The desired results are achieved either by comparing the measuring intensity to a constant intensity, which is obtained, for example, by covering the detector or by using a variable attenuator, by means of which the reference signal is adjusted so as to equal the measuring signal, and the position of the attenuator then indicates the amount of absorption. However, such measuring methods have their own sources of error, and it is necessary to note that the above-mentioned comparison of radiations traversing a reference cuvette and a measuring cuvette have been utilized for high-accuracy purposes. The comparison of the radiation along the different paths has been carried out by way of an interruptor frequency on the order of several hundred cycles per second, as a rule.
When it has been necessary to divide the radiation into two parts, certain drawbacks are encountered such as instability of the dividing objects, changing of the radiation pattern of the light source due to changing of the radiator temperature, for example, soiling of the sample cuvette, which evidences itself as a zero shift, changes of the mechanical dimensions of the dividing optics caused by the mechanical vibrations and ambient temperature changes and other similar causes. Elimination of these drawbacks requires frequently repeated calibration, and thus the known methods and apparatus are cumbersome for use in fields such as industrial processing.